1. Field of the Invention
The present invention relates to an elastic shaft joint used for a steering apparatus of an automobile and a method of forming an elastic bush thereof, and particularly to a technique for effectively absorbing displacement in the axial direction while securing comparatively high torsional rigidity.
2. Related Background Art
A steering apparatus of an automobile and the like generally comprises a steering wheel for a driver's steering, a steering gear for shifting the direction of dirigible wheels and a steering shaft for connecting the steering wheel and the steering gear. In such a steering apparatus, since the steering gear is rarely located on the axial center line of the steering wheel, a plurality of steering shafts connected by a universal joint are normally used. Such a universal joint for steering shafts is generally a Cardan joint in which a cross piece (joint cross) is movably mounted between a pair of joint elements, as disclosed in U.S. Pat. No. 3,501,928.
Recently, an elastic shaft joint with an elastic member of synthetic rubber or the like has been proposed in order to reduce transmission of kickback from the road surface to the steering wheel. In this type of elastic shaft joint, one joint element of a Cardan joint is separated into a joint member (yoke) and a shaft member (shaft), and an elastic ring of synthetic rubber, etc. is mounted between the yoke and the shaft. In order to prevent damage to the elastic ring and to enhance durability thereof, the yoke and the shaft are provided with stopper portions to limit more than a predetermined amount of relative rotation.
For example, Japanese Patent Application Laid-Open No. 6-329033 and Japanese Patent Application No. 11-324100 disclose an elastic shaft joint (first conventional device) wherein a comparatively shorter elastic bush is mounted between a yoke and a shaft. Also, Japanese Patent Application Laid-Open No. 9-229086 discloses an elastic shaft joint (second conventional device) wherein a pair of flat plate type elastic members is mounted between a yoke and a shaft. Further, Japanese Utility Model Application Laid-Open No. 4-69283 and Japanese Patent Application Laid-Open No. 8-200382 disclose an elastic shaft joint (third conventional device) wherein a comparatively longer elastic bush is mounted between a yoke and a shaft. Also, Japanese Utility Model Examined Publication No. 59-29147 and Japanese Patent Application Laid-Open No. 60-159418 disclose an elastic shaft joint (fourth conventional device) wherein two sets of elastic bushes are mounted separately away from each other between a cylindrical yoke and a cylindrical shaft. Furthermore, Japanese Patent Application Laid-Open No. 10-19054 discloses an elastic shaft joint (fifth conventional device) wherein an elastic member is disposed slidably in the axial direction in a corrugated cylindrical outer sleeve fitted on a yoke, and the outer sleeve is fixed to a shaft.
However, the above-mentioned elastic shaft joints have various properties and malfunctions caused by the properties. In the first conventional device, since only one shorter elastic bush is utilized, even though comparatively small force acts on the yoke and the shaft, the angle between the yoke and the shaft is easily changed, i.e., the yoke and the shaft are easily inclined to each other. Also, in the second conventional device, since the elastic members are not disposed circumferentially, the yoke and the shaft are easily inclined to each other in predetermined directions similarly to the first conventional device. Further, in the third conventional device, the longer elastic bush is utilized, so that the torsional rigidity can be kept comparatively high, but displacement of the yoke and the shaft becomes difficult in the axial direction. Further, in the fourth conventional device, the diameter of the elastic bushes is reduced. Therefore, when the strength of the elastic members is raised to secure the torsional rigidity, displacement of the yoke and the shaft becomes difficult in the axial direction. Furthermore, in the fifth conventional device, in order to slide the outer sleeve and the elastic member smoothly without looseness, high accuracy is required to form the outer sleeve and the elastic member. In addition, the yoke and the shaft are easily inclined similarly to the first and second conventional devices.
In the first, second and fifth conventional devices, due to easy inclination of the elastic shaft joint, when a driver turns a steering wheel from right to left, it is difficult to have torque transmitted to a steering gear, resulting in bad response of the steering system lacking in feeling of rigidity. In addition, excessive inclination of the elastic shaft joint causes large tensile and compressive deformation of the elastic member, leading to shortening of the lifetime of the device. Furthermore, in the third conventional device, preferable results can be obtained in respect to torsional rigidity and nonoccurrence of easy inclination. However, due to the difficult displacement in the axial direction, vibration from the steering gear is transmitted to the steering wheel and noise tends to be generated.
On the other hand, in the fifth conventional device, as the two sets of elastic bushes are provided away from each other in the axial direction, the inclination of the yoke and the shaft can be nearly neglected. However, the elastic bushes are disposed in the cylindrical yoke, so that when the diameter of the elastic bushes is increased, the weight of the device increases largely. On the other hand, when the diameter of the elastic bushes is kept small and a material with high strength is used for the elastic members, stability of response in steering is improved, but displacement in the axial direction is difficult, similar to the third conventional device.